Protecting aluminum from corrosion



Patented Feb. 6, 1934 uurrso STATES FROM ooaaosron Junius D. Edwards, Oakmont, Pa assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing.

Application April 29, 1933, Serial No. 668,630, and in Canada June 20, 1931 11 Claims. (Cl. 148-8)v This invention relates to the covering of the surfaces of aluminum andaluminum base alloys, herein collectively designated as aluminum, with a coating capable of resisting the action of corrosive agencies. The invention particularly contemplates the provision on said aluminum surfaces of a corrosion resistant oxide coating.

By various known processes aluminum surfaces may be provided with what is generally termed an "oxide coating. Such a coating is composed in large part of aluminum oxide. stated, the coatings may be produced by various methods, which methods, with few exceptions, comprise a reaction, chemical or electro-chemical, between the aluminum surface and a solution of a chemically active substance or substances. The present invention refers to coatings which are produced by these artificial means and which are adsorptive. It is a further desirable characteristic of such coatings that they be comparatively heavy or thick, but a comparatively thin coating may be satisfactory, especially if its adsorptive power is high. The very thin film of oxide naturally forming on aluminum surfaces is not herein referred to as an oxide coating.

One definite use of these oxide coatings is to protect the aluminum surfaces against the action of corrosive agencies. In the performance of this function the oxide coatings find an important commercial justification. It is an important ob- -ject of this invention to provide methods whereby the protection against corrosion offered by these oxide coatings formed on aluminum surfaces may be substantially increased and whereby the corrosion resistant quality of the oxide coating may be so developed that the coated article has an increased useful life. A further object of the invention is the provision of oxide coated aluminum objects which may be used successfully under severe corrosive conditions.

To these and other ends the invention comprises the production of corrosion resistant coatings by forming adsorbent oxide coatings on aluminum surfaces and thereafter impregnating said coatings by treating them with a solution of soluble silicate. The oxide coating is first formed on the aluminum surface. For instance the aluminum is made the anode of an electrolytic cell, the cathode of which may be aluminum, lead or other suitable metal, with an electrolyte of sulphuric acid in aqueous solution. External electrical energy is impressed on the cell with the result that a layer of oxide is formed on the surface of the aluminum anode. This oxide coating is adsorptive and may be of varying thickness. Or the aluminum may be immersed in hot solution of sodium carbonate and alkali dichromate for a short period of time with the resulting formation of an adsorptive oxide coating on the surfaces thereof. Other methods of forming these oxide coatings are well known, and may be used, but I prefer the sulphuric acid method outlined above. I

The aluminum, having been provided with an adsorptive oxide coating, is preferably washed to remove residual solutions remaining from the coating process and is then ready for the second step of the process, which consists in treating the oxide coated aluminum surface with a solution containing soluble silicate. Simple immersion is the most feasible method of treatment, and in commercial practice the oxide coated aluminum is immersed in the solution of soluble silicate, but other methods of conveying the solution to the surface will give good results. As a result of these treatments the soluble silicate is adsorbed inand on the adsorptive oxide coating. The oxide coated aluminum is then'rinsed, if desired, and thoroughly dried.

A number of soluble silicates are available, but they vary in their composition and suitability for my process. The silicate which has a. silica to soda ratio of about 1:1 (1. e., one molecule of silica to one of soda) is too alkaline for generally satisfactory use, and even short immersion of oxide coats in a 5 per cent solution will produce appreciable attack at room temperature and rapid attack at boiling temperature. If, however, the silica to soda ratio is about 2:1 or higher, satisfactory results can be obtained. With silicate having a ratio of 2:1, the concentration or the temperature of impregnation should not be too high if complete attack is to be avoided. For

this reason I prefer to use a silicate solution which has a silica to soda ratio of about 3:1 or higher, particularly when a boiling solution is employed. In using potassium silicate I prefer a silicate having about a 4:1 ratio, though silicates of lower ratio may be used, as in the case of sodium silicate. The ratios given here are approximate,

corresponding generally to the ratios in commercial products. Thus commercial silicates having ratios of 3.86:1 and 3.25:1 or 2.84:1 would be close enough to 4:1 or 3:1 for satisfactory use in my process. The concentration of the solution may vary within wide limits, from very dilute, such as a per cent or two upward, but concentrations around 30 per cent are thick, viscous liquids and hence are diflicult to handle and are adsorbed slowly. Less trouble is met at 20 per cent, but I have not observed any advantage in going above about 5 per cent, which is the preferred strength.

The final product of my process has exceptional properties. Its resistance to the action of corrosive agents, even of a severe character, is decidedly greater than the resistance of an oxide coated aluminum which has not been treated with a solution of soluble silicate in accordance with my invention. The silicate absorbed in the oxide coating is, despite its soluble nature,

held firmly therein and is leached out only slowly by water as evidenced by the fact that oxide coated aluminum surfaces which have been treated in solutions of soluble silicate held at ordinary temperatures do not lose their highly resistive property under prolonged exposure to the action of spray from a 20 per cent salt solution. For instance a group of aluminum alloy specimens were oxide coated electro-chemically in sulphuric acid solution and a part of the group were then treated with soluble silicate in accordance with my invention. Both the treated and untreated oxide coated specimens were exposed for 52 weeks to the action of a spray of a 20 per cent salt solution. Those specimens treated in accordance with my invention lost only 5.3 per cent, on the average, of their elongation, whereas the oxide coated specimens which were untreated lost, on the average, 16 per cent of their elongation.

Although such results are excellent and superior they can, I have found, be decidedly improved upon when the oxide coated aluminum.

surface is subjected to certain special treatment.

I have discovered that while highly effective results are obtained when the oxide coated aluminum surface is treated with solutions of soluble silicate held at any temperature there is, nevertheless, a range of temperature through which the treatment is most effective,-namely when the solution of soluble silicate is held at temperatures above about 80 C. and up to and including the boiling temperature of the solution. As the result of such a treatment the soluble silicate is adsorbed in and on the oxide coatingand the pores of the coating are apparently closed so that,

under the action of weather, moisture and similar agents the removal of any important amount of silicate is practically precluded.

As illustrative of the improved results obtained by practicing my invention at the elevated temperatures aforesaid I may cite the case of duralumin, coated anodically in a sulphuric acid electrolyte and after washing immersed for 30 minutes in a boiling solution of sodium silicate containing about 5 per cent of silicate having a silica to soda ratio of about 3.25:1. The rinsed and dried metal was subjected for 52 weeks to the salt spray test and at the end of that period was practically free from signs of corrosive attack.

This application is a continuation in part of my application for United States Letters Patent, Serial No. 613,789, filed May 26, 1932, and a continuation in part of my application for United States Letters Patent, Serial No. 472,744, filed August 2, 1930.

I claim- 1. In a method of treating aluminum articles to render the surface thereof resistant to prolonged exposure to corrosive influences, forming on the surface of the article an adsorbent oxide coating, and thereafter adsorbing silicate in said coating from a. silicate solution, said silicate after adsorption being resistant to solution by washing with water.

2. In a method of treating aluminum articles to render the surface thereof resistant to prolonged exposure to corrosive influences, forming on the surface of the article an adsorbent oxide coating, and thereafter adsorbing silicate in said coating from a silicate solution maintained at a temperature above about 80 C., said silicate after adsorption being resistant to solution by washing with water.

3. In a method of treating aluminum articles to render the surfaces thereofresistant to prolonged exposure to corrosive influences, forming an adsorbent oxide coating on the surface of the article by anodic oxidation in an electrolyte, and thereafter adsorbing sodium silicate in said coating from a solution of sodium silicate in which the ratio of silica to soda is not less than about 2 to 1. I

4. In a method of treating aluminum articles to render the surface thereof resistant to prolonged exposure to corrosive influences, forming on the surface of the article by anoclic oxidation in an electrolyte containing sulphuric acid an adsorbent oxide coating, and thereafter adsorbing sodium silicate in said coating from a solution of sodium silicate in which the ratio of silica to soda is not less than about '2 to 1.

5. The method of producing corrosion resistant coatings on aluminum by forming on the surface thereof an adsorbent oxide coating and thereafter treating said coating with-a potassium silicate solution in which the ratio of silica to potash is not less than about 2 to 1.

6. In a method of treating aluminum articles to render the surface thereof resistant to prolonged exposure to corrosive influences, forming on the surface of the article by anodic oxidation in an electrolyte containing sulphuric acid an adsorbent oxide coating, and thereafter adsorbing potassium silicate in said coating from a solution in which the ratio of silica to potash is not less than about 2 to 1.

7. The method of producing corrosion resistant coatings on aluminum by forming on the surface thereof an adsorbent oxide coating and thereafter treating said coating with a sodium silicate solution in which the ratio of silica to soda is not less than about 2 to 1, the said solution being maintained during treatment at a temperature above about 80 C.

8. The method of producing corrosion resistant coatings on aluminum by forming on "the surface thereof an adsorbent oxide coating and thereafter treating said coating with a sodium silicate solution in which the ratio of silica to soda is not less than about 3 to 1, the said solution being maintained during treatment at a temperature above about 80 C.

9. As a new article of manufacture, an article of metallic aluminum having its surface rendered resistant to prolonged exposure to corrosive influences by the presence on such surface of an adsorbent oxide coating impregnated with a silicate adsorbed and strongly bound therein.

10. As a new article of manufacture, an article of metallic aluminum having its surface rendered resistant to prolonged exposure to corrosive influences by the presence on such surface of an adsorbent oxide coating impregnated with sodium silicate and strongly bound therein.

11. As a new article of manufacture, an article of metallic aluminum having its surface rendered resistant to prolonged exposure to corrosive influences by the presence on such surface of an adsorbent oxide coating impregnated with potassium silicate and strongly bound therein.

JUNIUS I EDWARDS. 

